1. Field of the Invention
The present invention relates to a vertical cavity surface emitting laser.
2. Description of the Related Art
A Vertical Cavity Surface Emitting Laser (VCSEL), which emits a laser beam in a direction vertical to a substrate, has advantages of such as being easily two-dimensionally arrayed into high density.
By using the VCSEL array in which these vertical resonator type lasers are accumulated in high density, an electrophotography can form an image of a higher definition with a higher speed.
One problem of the VCSEL is a transverse mode control, and U.S. Pat. No. 6,144,682 proposes a structure (surface relief) in which a shallow relief is formed on the top surface of a Distributed Bragg Reflector (DBR) and thereby forms a distribution of reflectance in the plane.
FIG. 6 illustrates an outline of the structure disclosed in U.S. Pat. No. 6,144,682.
In FIG. 6, a semiconductor DBR 1302 is stacked on a substrate 1301, and a resonator 1304 containing a plurality of quantum wells 1313, and a semiconductor DBR 1303 are formed thereon. A selectively oxidizable layer 1316 having a high Al composition is formed in this semiconductor DBR 1303.
An oxidized part 1317 for confining an electric current and an unoxidized part 1318 are formed by oxidizing the selectively oxidizable layer 1316 from a side wall of mesa structure.
A contact layer 1320 for electrically contacting an upper electrode 1321 is formed on the top layer of the semiconductor DBR 1303.
A lower electrode 1322 is provided on the rear surface of the substrate 1301.
A region 1323 is formed by etching one part of the contact layer 1320. Due to this etching, the reflectance of the circular region 1323 which has been etched becomes higher than that of an unetched part.
Thereby, the reflectance in the central part of the surface emitting laser becomes higher than that in the peripheral part. As a result, the oscillation of a high-order transverse mode can be reduced.
In addition, Chirovsky et al., IEEE, PTL Vol. 11, 1999, pp 500 proposes an intracavity type surface emitting laser which has a dielectric DBR stacked on a semiconductor DBR and injects an electric current from the semiconductor DBR.